1. Field
The present disclosure relates to a process and associated methods to harden a hardfacing weld overlay alloy. More specifically, the present disclosure relates to a heat-treatment process to harden the weld overly of an iron-chromium-carbide hardfacing alloy to significantly improve the resistance of the weld overlay against erosion, abrasion, and erosion-corrosion.
2. General Background
Fe—Cr—C alloy system is a well known hardfacing material. Carbon is needed to form hard particles of carbide to contribute the alloy's resistance to erosion or abrasive wear. More carbon in the alloy forms more volume fraction of carbides, thus exhibiting more resistance to wear. Thus, common hardfacing alloys of this type contain more than 2% carbon. Chromium is added to the alloy to form much more stable chromium carbides instead of less stable iron carbides (if no chromium in the alloy). Chromium is also useful in increasing the alloy's oxidation resistance by forming chromium oxides when the component is intended for services at high temperatures. This group of hardfacing alloys is often referred to as “high-alloy white cast irons”. General discussion of this group of hardfacing alloys can be found in ASM Handbook, Vol. 4, Heat Treating, p. 700. The alloys are typically used in forms of castings or hardfacing weld overlays. The large volume of eutectic carbides in the microstructure of a casting or weld overlay provides high hardness for abrasion resistance.
Alloys of various compositions in this group are also subject to heat treatments to produce additional hardening by forming martensite in the alloy. This martensitic phase transformation is a well known phase transformation in Fe—C alloy system by heating the alloy at a high temperature in an austenitic phase range followed by fast cooling to a temperature below the critical temperature, typically referred to as Ms temperature (i.e., the temperature when the martensite phase starts forming at the temperature when the metal is being cooled to room temperature. The hardness of the alloy will significantly be increased when the microstructure of the alloy contains martensite. The Ms temperature varies depending on the composition of the alloy. Some high chromium alloys exhibit such very low Ms temperatures that the alloys have to be cooled well below room temperature in order to produce additional hardening by forming martensite. These alloys are to be refrigerated in order to transform the austenite phase to martensite phase for additional hardening. Typical of such alloys are those described in U.S. Pat. Nos. 3,941,589, 4,547,221, and 5,183,518. U.S. Pat. No. 3,941,589 describes alloy composition comprising 2.5-3.5% carbon, 2.5-3.5% manganese, 12-22% chromium, 1-2% silicon, 1.5-3.0% molybdenum, 1-2% copper, and balance iron. The alloy of this referenced invention is hardened by transformation of some austenite to martensite by a refrigeration heat-treatment involving cooling the metal to a temperature usually below about −100° F. (−75° C.) for a period of time. U.S. Pat. No. 4,547,221 describes alloy composition comprising about 2.6-3.6% carbon, about 12-22% chromium, about 0.5-1.1 manganese, about 1.0-3.0% molybdenum, about 0.5-1.5% copper, about 1.4-2.5% nickel, about 1.4-2.5% silicon, and balance iron. The alloy of this invention is also hardened by a refrigeration heat-treatment involving cooling the metal to a temperature usually below −100° F. (−75° C.) for a period to allow additional austenite to transform to martensite. U.S. Pat. No. 5,183,518 describes alloy composition comprising 2.4-3.8% carbon, 0.4-2.0% manganese, 0.2-1.9% silicon, 0.0-3.0% copper, 1.5-4.5% nickel, 12.0-29.0% chromium, and the remainder iron. The alloy of this invention is hardened by cooling the metal to a cryogenic temperature of about −55° C. (a temperature well below the Ms temperature for the alloy) for a sufficient time to form martensite. Some Fe—Cr—C hardfacing alloys have a much higher Ms temperature, which allows formation of martensite when cooled to room temperature. Typical of such alloys is described in U.S. Pat. No. 6,375,895. U.S. Pat. No. 6,375,895 describes alloy composition comprising about 0.65-1.1% carbon, about 4.5-10.5% chromium, about 0.05-1.0% molybdenum, and balance iron. This hardfacing alloy is suited for welding on the surfaces for protection from abrasion wear. The alloy weld metal can be hardened by forming martensite when cooled down to room temperature.
It is well known that the martensite phase forms when a high-temperature austenite phase in a face-centered cubic structure of steel is cooled to a temperature below Ms temperature to form martensite having a body-centered tetragonal structure with all the carbon atoms being trapped in the structure that produces severe strain in the martensite. As a result, a significant hardening is produced in the metal when martensite is formed. The martensite is not thermally stable. This means when the metal is heated to above Ms, which is the temperature martensite starts to form when the metal is being cooled to lower temperatures from an austenitizing temperature, the trapped carbon atoms in the martensite diffuse away from a highly distorted body-centered tetragonal structure that turns into a regular, non-distorted body-centered cubic structure, thus eliminating all the strain in the metal and losing the hardening. The Ms temperature, depending on the alloy chemistry, can be very low for some alloys. For example, Ms temperature of the alloy comprising 2.4-3.8% carbon, 0.4-2.0% manganese, 0.2-1.9% silicon, 0.0-3.0% copper, 1.5-4.5% nickel, 12.0-29.0% chromium, and the remainder iron is below 150° C. (U.S. Pat. No. 5,183,518). Accordingly, the metal that is hardened by martensite cannot maintain its abrasive wear resistance when exposed to elevated temperatures. Furthermore, the high hardness produced by martensite formation is the result of severe strain produced by a distorted crystal structure, not by hard particle phases. Hardness produced this way is not known to exhibit resistance to erosion by the particles-entrained flue gas streams generated in many industrial environments, such as boilers or petrochemical processing.
High alloy white cast irons, which typically contain more than 2% carbon along with chromium and other alloying elements as discussed earlier, contain a large volume of eutectic carbides that provide abrasive wear resistance. These alloys are normally used in castings for machinery in crushing, grinding and other applications for handling abrasive materials. When these alloys are used as a hardfacing, such as a weld overlay, on a metallic component to resist abrasive wear, the weld overlay can develop stress cracks due to large volume of eutectic carbides. In some industrial applications, these stress cracks in the weld overlay may not present performance or safety related issues. However, in some other applications involving pressure boundary components, such as boilers and vessels as well as piping, the weld overlay on these components is to be free of stress cracks. The alloys that are suitable for applications as a weld overlay for these critical components would require a composition containing lower carbon content with lower volume of eutectic carbides. This will allow the use of welding process to produce a hardfacing weld overlay without developing stress cracks. However, when the volume of eutectic carbides is reduced as a result of lowering carbon content, the alloy's wear resistance is also reduced because of lower hardness. It becomes important that a novel heat-treatment method be developed to further harden a crack-free weld overlay to significantly improve the overlay's resistance to abrasive, erosion wear.
HF35 is a hardfacing alloy comprising about 0.8-1.2% carbon, about 20-23% chromium, about 2.5-3.5% nickel, about 0.2-0.5% zirconium, about 0.5-1.0% molybdenum, about 1.0-2.0% manganese, about 1.0-2.0% silicon, and balance iron along with impurities and incidental elements. The alloy contains much lower carbon as compared with high-alloy white cast irons and other Fe—Cr—C eutectic carbide alloys. The level of chromium in the alloy is (a) to form more stable eutectic chromium carbides (instead of eutectic iron carbides if no or low chromium in the alloy) and (b) to form chromium oxide scales when used at high temperatures to improve oxidation resistance in order to improve the alloy's resistance to erosion/corrosion. Nickel of about 3% is to increase the stability of austenite and improve the alloy's toughness. Additions of other alloying elements, such as molybdenum and zirconium, are intended to further improve the alloy's abrasion, erosion, and erosion/corrosion resistance. Due to much lower carbon content, the volume of eutectic carbides is much reduced, thus resulting in lower hardness. When the alloy is weld overlaid on a component, such as tube, pipe, vessel, or boiler waterwall, the overlay does not develop cracks. However, the alloy's resistance to abrasion or erosion wear is compromised because of its lower hardness. The hardness for the weld overlay of this hardfacing is typically RC 35-40 in the as-overlaid condition.
A hardfacing alloy with hardness of about RC 35-40 is generally considered to be resistant to moderately abrasive and erosive environments. For highly abrasive and erosive conditions, such hardfacing alloy with hardness of about RC 35-40 is not likely to perform well. For example, HF35 overlay tubes were tested as part of the in-bed evaporator tube bundle in a fluidized-bed coal-fired boiler that generates electricity. The overlay tubes were tested for about three years. Two tubes were then removed for evaluation. The examination showed that the HF35 overlay performed well for most of the tube except some localized areas that the overlay was worn off. This localized area was apparently subject to high abrasive and erosive conditions and the HF35 weld overlay, with about RC35-40, was found to be inadequate.
An existing Fe—Cr—C hardfacing alloy weld overlay that can be weld overlaid to a part without stress cracks exhibits only moderate hardness. Thus, there is a need to develop a novel method to further increase the hardness of this moderately hardened hardfacing weld overlay to a level, such as RC50 or higher, such that the weld overlay's resistance to abrasive and/or erosive wear becomes adequate for use in aggressive abrasive and erosive environments.
In a test program trying to determine whether the HF35 overlay would be susceptible to cracking when the overlay was heated to very high temperatures, such as 2000° F., an HF35 overlay tube sample was furnace heated to 2000° F. and held for about one hour followed by furnace cooling to 1600° F. and then removed from the furnace and air cooled to room temperature. It was unexpectedly discovered that the overlay, which exhibited hardness of RC40 before this heat-treatment, was hardened to RC54 after this heat-treatment. This was a significant increase in hardness for the weld overlay produced by this simple heat-treatment. It was also discovered that this heat-treatment did not cause cracking of the hardfacing weld overlay. To see whether air cooling from 1600° F. to room temperature was responsible for this hardening, a sample of another HF35 overlay tube was placed in a 1600° F. furnace and the temperature was increased to 2000° F. by a furnace heat-up. The sample was held at 2000° F. for one hour and then furnace-cooled to 1600° F. and then continued to room temperature by furnace cooling. Significant hardening was also observed by this very slow furnace cooling. Hardness was increased from RC38 in the as-overlaid condition to RC54 after this heat-treatment with very slow furnace cooling. Thus, the hardening was not the result of well-known phase transformation to martensite during cooling to room temperature.